Electrical Power System for a Subsea System

ABSTRACT

An electrical power system for stationary or movable subsea loads ( 7 ) providesone common feeder for multiple electric motors which can be individually controlled. Operational flexibility and operational safety for operation in varying water depths is provided by encapsulating electrical functional element ( 6 ) of a subsea power system with a subsea electrical distribution system ( 5 ) individually or in-groups. Electrical functional element ( 6 ) and their semi-conductor elements are arranged within at least one fluidized internal pressure casing ( 13 ). An external pressure casing ( 12 ) is provided for the subsea electrical distribution system ( 5 ) and/or other components of the subsea system. A high frequency power transmission to the subsea pressurized distribution system ( 5 ) with pressurized semi-conductor components reduces weight and size of subsea transformers ( 4   b ) and cables ( 9 ) employed in subsea systems ( 10 ).

The present invention relates to an electrical power system for a subseasystem. The invention also relates to a method for operating at leastone electrical load, e.g. an electrical motor, in a subsea application.Furthermore the invention also relates to a subsea remotely operatedvehicle.

A subsea system may be for example a subsea oil field installation or asubsea remotely operated vehicle (ROV). Remotely operated vehicles(ROVs) are mostly unmanned and are used extensively for the inspectionand maintenance of subsea oil field installations. Subsea systems mayalso be employed for seabed mining. Subsea installations for subsea oilfield or other submarine applications, in particular applicationsinvolved with the exploration of subsea resources, may be fed by apossibly large umbilical which usually contains one or more power supplycables and at least one control cable. Subsea systems and ROVs inparticular are usually powered by high voltage electricity or byhydraulic oil.

Electrical components of the subsea system have to be isolated andprotected against sea water and pressure at deep sea levels. Thereforeknown subsea systems may comprise a vessel pressurised at 1 atmosphere.The housing of such pressurised vessels is often very heavy weight andthus limiting the manoeuvrability of the subsea system. Reducing theweight of the housing in existing systems may lead to less protectionand increased likelihood of damages. Such risks increase when the subseasystem is operating in deep waters or at altering depths.

It is an object of the present invention to provide an electrical powersystem for a subsea system which avoids or reduces the disadvantages ofthe prior art and increases the manoeuvrability and operationalflexibility of a subsea system with an electrical power system.

According to the present invention this is achieved by an electricalpower system for a subsea system comprising at least one subsea powerdistribution system receiving power from a power source, said subseapower distribution system comprising at least one electrical functionalcomponent, and at least one connecting member for at least oneelectrical load, e.g. a propulsion system or a motor for subseaoperation, wherein an external pressurised casing is provided for thesubsea power distribution system, and wherein at least one internalpressurised casing is provided for the at least one electricalfunctional component. This arrangement increases the ease of handling,enables a low weight design of the subsea system and facilitates ahigher degree of standardisation of the modules of the subsea system andof the electrical power system. According to the invention two-stagepressure compensation is possible.

Advantageously the subsea power distribution system may comprise aplurality of electrical functional components and at least one internalpressurised casing may be provided for pressurising at least oneelectrical functional component or at least one of its parts.

Advantageously the at least one internal pressurised casing may befluidised.

In order to provide further protection for the electrical functionalcomponents the at least one internal pressurised casing may be at leastpartly filled with a liquid.

In order to minimize thermal losses and at the same time provideefficient pressure compensation the at least one internal pressurisedcasing may be at least partly filled with oil or a liquid substance withoil as one of its components.

Advantageously the external pressurised casing may be at least partlyfilled with a gas or a mixture of gases. This may be particularlyfavourable for shallow water use.

Advantageously the external pressurised casing may be at least partlyfilled with nitrogen.

The internal pressurised casing of the electrical functional componentsresults as especially advantageous if at least on electrical functionalcomponent comprises semi-conductor elements.

Advantageously at least one electrical functional component comprisingsemi-conductor elements may be a cycloconverter.

Advantageously at least one semi-conductor element may be a thyristor.

Advantageously one electrical functional component comprisingsemi-conductor elements, said electrical functional component beingarranged within an internal pressurised casing, may be provided for eachconnecting member of the subsea system.

Advantageously at least one electrical functional component comprisingsemi-conductor elements, said electrical functional component beingarranged within an internal pressurised casing may be provided forconnection to the power source.

Advantageously at least one connecting member for at least oneelectrical load may be a subsea plug.

Advantageously the subsea power distribution system may be static.

Advantageously the electrical power system may comprise a subseaelectrical power system according to the invention or according to oneor more of its embodiments, at least one topside converter providing anoutput frequency of at least 100 Hz to be transmitted to the subseasystem and at least one cable for power transmission to the subseasystem, said cable being connected to the topside converter and saidcable being connected to the subsea system. By using high frequencypower transmission to the subsea system, weight and inductance of thepower transmission cable may be significantly reduced. Furthermore thespace consumption and weight of the electrical equipment used withinsubsea may also be reduced, in particular the use of more light weightedand smaller transformers is enabled. Manoeuvrability and operationalflexibility of the subsea system may be increased by using highfrequency power transmission.

Advantageously the output frequency of the converter may be at least 200Hz.

Advantageously the output frequency of the converter may be at least 300Hz.

Advantageously the output frequency of the converter may be at least 380Hz.

The present invention also provides a method for operating at least oneelectrical load in a subsea application using an electrical power systemaccording to the invention or according to one of its embodiments forpower transmission to a subsea power distribution system.

The present invention also provides a subsea remotely operated vehicle(ROV) with an electrical power system according to the invention oraccording to one or more of its embodiments with at least one electricalload being a propulsion system for the subsea remotely operated vehicle,said propulsion system receiving power from the subsea powerdistribution system.

Further preferred features, details and advantages of the invention willnow be described by way of example with reference to the accompanyingdrawing, in which:

FIG. 1 is a schematic view of an electrical power system for a subseasystem.

FIG. 1 shows in schematical view an electrical power system for a subseasystem 10. The subsea system itself is shown in an abstract, schematicalview. This view is focused on the overall electrical design and is notintended to be comprehensive.

The subsea system 10 may be a remote operated vehicle (ROV) for subseaoperation. Such ROVs are usually unmanned and may be built capable tooperate in shallow and in deep water with water depths deeper than 1000meter and up to 3000 meter, 5000 meter and more. The subsea system 10comprises or may be connected to at least one electrical load 7. In theexample shown the electrical loads 7 are electrical motors. Suchelectrical motors may be used for propulsion of the subsea system and/orfor manipulators and/or controllers for subsea applications.

The electrical power system of the subsea system 10 comprises a powerdistribution system 5. The subsea power distribution system 5 compriseselectrical functional elements 6, preferably at the input side and/or atthe output side of the power distribution system 5. In order to providea connection, which is safe in operation under submarine conditions,subsea plugs 8 are used as connecting members for connecting theelectrical loads 7 to the subsea power distribution system 5 and to theelectrical functional elements 6.

The subsea system 10 may be stationary or mobile. The subsea electricalpower system of the subsea system 10 may be connected to electricalloads 7, which are mechanically attached to or that form at leasttemporarily part of the subsea system 10. The subsea electrical powersystem of the subsea system 10 may also be connected to electrical loads7, which are part of other stationary or mobile subsea installations. Itis possible that the electrical loads 7 may be connected and/ordisconnected from the power distribution system 5. Electrical loads 7may operate in pump systems, such as booster pumps or water injectionpumps, which may be used in oil field or mining applications on the seabed.

Power for the subsea system 10, e.g. an oil field subsea installation ora ROV, is fed from a top side power system 3 using at least one cable 9.The top side power system 3 is usually located above sea-level 11. Thetop side power system 3 may also be located at about sea-level 11 or atleast partly below sea-level 11. The top side power system 3 maycomprise a shore-sea cable, which is not specifically shown in FIG. 1,and/or one or more generators 1. The top side power system 3 may belocated on a platform. The top side power system usually operates at afrequency of about 50 Hz or about 60 Hz.

In the embodiment shown by way of example, at least one converter 2 isprovided between the top side power system 3 and the at least one powercable 9 for transmitting power for the subsea system 10. The converter 2is preferably a high frequency converter which is designed to convertthe lower frequency power of the power system 3 to a high frequency, forexample to a frequency within the range of about 100 Hz to about 400 Hz.The cable 9 is designed for high frequency power transmission from a topside power system 3 to a subsea system 10. One or more powertransmission cables 9 may be arranged in an umbilical connecting thesubsea system 10 and its power distribution system 5 to a top sideinstallation. A top side installation may be for example a platform, avessel or a shore-sea cable. Said umbilical may also comprise one ormore control cables for one or more subsea system 10 and/or connectedloads 7.

At the top side of the at least one cable 9, a transformer 4 a may beprovided. At least one transformer 4 b may also be provided at thesubsea side. When high frequency power transmission to the subsea system10 is used, the transformer 4 b, which is comprised by the subsea system10 may be considerably lighter than transformers that were previouslybeing used for subsea systems 10.

Electrical functional elements 6 may be arranged between the powerdistribution system 5 and the connection to a power source, e.g. the topside power system 3. Electrical functional elements 6 may also bearranged between the power distribution system 5 and electrical loads 7for subsea operation. Subsea plugs 8 may be used as part of connectingmembers. Electrical functional elements 6 may operate for example asswitches and/or converters. The power distribution system 5 ispreferably a static power distribution system without moving parts.

Preferably the electrical functional elements 6 comprise semi-conductorelements, which may operate as breaker, soft-start control and/orfrequency control for a subsea process load 7, i.e. an electricalconsumer, e.g. an electrical motor.

The electrical functional elements 6 are pressure compensated by the useof an internal pressurised casing 13. Other parts of the subsea powerdistribution system 5 and/or the subsea system are pressurised using anexternal pressurised casing 12. Preferably for each electricalfunctional element 6, a group of electrical functional elements 6, or atleast for the semi-conductors comprised by an electrical functionalelement 6 an individual internal pressurized casing 13 is provided.

An electrical functional element 6 and/or its semi-conductor componentsare enclosed in a liquid within an internal pressurised casing 13.Preferably said liquid consists at least in part of oil. The externalpressurised casing 12 is preferably filled at least partly with a gas ora mixture of gases, e.g. nitrogen. In this way a two-stage pressuresystem for the subsea electrical power system of the subsea system 10may be provided.

A subsea power distribution system 5 is provided with electricalfunctional elements 6, which operate as multi functional, reliablecontrollers for electrical power loads 7 to be installed at variouswater depths from shallow to ultra deep water. The external pressurisedcasing 12 may be designed as a canister having at least in part aprimarily cylindrical form.

The electrical functional elements 6 comprising semi-conductor elementsmay provide direct online start, soft start, i.e. low torque start, andvariable frequency control for multiple electrical loads 7. Reversing ofthe motors may be included in the control. An electrical functionalelement 6 may also operate as direct driver.

An electrical functional element 6 may comprise a cycloconverterconnection, preferably with branch fuses, or a star connection, whichmay be fuseless. It is of advantage for subsea applications if thenumber of cables per phase leading to an electrical load 7 is limited.One or more electrical components 6 can be installed inside one internalpressurised casing 13, e.g. for providing the functionality of acycloconverter

An electrical functional element 6 may comprise at least one, preferablya plurality of thyristors as semi-conductor elements, in particular whendesigned as a static switching element. One or more thyristors may beused in a breaker, a soft-starter and/or a cycloconverter.

The subsea electrical power system provides preferably an output rangefrom about 3 MVA to about 30 MVA. Electrical functional elements 6 maybe arranged in open or in star connection. The supply voltage of thesubsea electrical power system may be for example of about 1180V,controlled and with isolated motor phases. If designed for a high numberof electrical loads 7, which may be arranged in serial, a higher supplyvoltage may be preferred. A high short-time overload capability, e.g.200% for 60 seconds, is provided.

The range of a movable subsea system 10 may be narrower when using highfrequency power transmission but its operational flexibility isaugmented due to lighter and more simple construction and design.

A primary aspect of the invention may be summarized as follows:

The invention relates to an electrical power system for stationary ormovable subsea loads 7 providing one common feeder for multiple electricmotors which can be individually controlled. A higher operationalflexibility and increased operational safety for operation in varyingwater depths is provided by encapsulating electrical functional elements6 of a subsea power system with a subsea electrical distribution system5 individually or in groups. Electrical functional elements 6 and theirsemi-conductor elements are arranged within at least one fluidisedinternal pressure casing 13. Additionally an external pressure casing 12is provided for the subsea electrical distribution system 5 and/or othercomponents of the subsea system. In addition or alternatively to thearrangement described above, employing high frequency power transmissionto the subsea pressurized distribution system 5 with pressurisedsemi-conductor components may enable a reduction of weight and size ofsubsea transformers 4 b and cables 9 employed in subsea systems 10.

1. Electrical power system for a subsea system (10) comprising at leastone subsea power distribution system (5) receiving power from a powersource, said subsea power distribution system (5) comprising at leastone electrical functional component (6), and at least one connectingmember for at least one electrical load (7) for subsea operation,characterised in that an external pressurised casing (12) is providedfor the subsea power distribution system (5), and that at least oneinternal pressurised casing (13) is provided for the at least oneelectrical functional component (6).
 2. Electrical power systemaccording to claim 1, characterised in that the subsea powerdistribution system (5) comprises a plurality of electrical functionalcomponents (6) and that at least one internal pressurised casing (13) isprovided for pressurising at least one electrical functional component(6) or at least one of its parts.
 3. Electrical power system accordingto claim 1 or 2, characterised in that the at least one internalpressurised casing (13) is fluidised.
 4. Electrical power systemaccording to claim 3, characterised in that the at least one internalpressurised casing (13) is at least partly filled with a liquid. 5.Electrical power system according to claim 4, characterised in that theat least one internal pressurised casing (13) is at least partly filledwith oil or a liquid comprising oil.
 6. Electrical power systemaccording to one of claims 1 to 5, characterised in that the externalpressurised casing (12) is at least partly filled with a gas or amixture of gases.
 7. Electrical power system according to claim 6,characterised in that the external pressurised casing (12) is at leastpartly filled with nitrogen.
 8. Electrical power system according to oneof claims 1 to 7, characterized in that at least one electricalfunctional component (6) comprises semi-conductor elements. 9.Electrical power system according to claim 8, characterized in that atleast one electrical functional component (6) comprising semi-conductorelements is a cycloconverter.
 10. Electrical power system according toclaim 8 or 9, characterised in that at least one semi-conductor elementis a thyristor.
 11. Electrical power system according to one of claims 8to 10, characterised in that one electrical functional component (6)comprising semi-conductor elements, said electrical functional componentbeing arranged within an internal pressurised casing (13), is providedfor each connecting member of the subsea system.
 12. Electrical powersystem according to one of claims 8 to 11, characterised in that atleast one electrical functional component (6) comprising semi-conductorelements, said electrical functional component being arranged within aninternal pressurised casing (13), is provided for connection to thepower source.
 13. Electrical power system according to one of claims 8to 12, characterised in that the at least one connecting member for atleast one electrical load (7) is a subsea plug.
 14. Electrical powersystem according to one of claims 8 to 13, characterised in that thesubsea power distribution system (5) is static.
 15. Electrical powersystem comprising a subsea electrical power system according to one ofclaims 1 to 14, characterised in that it comprises at least one topsideconverter (2) providing an output frequency of at least 100 Hz to betransmitted to the subsea system (10) and that it comprises at least onecable (9) for power transmission to the subsea system (10), said cablebeing connected to the topside converter (2) and said cable beingconnected to the subsea system (10).
 16. Electrical power systemaccording to claim 15, characterised in that the output frequency of theconverter (2) is at least 200 Hz.
 17. Electrical power system accordingto claim 15 or 16, characterised in that the output frequency of theconverter (2) is at least 300 Hz.
 18. Electrical power system accordingto claim 15, 16 or 17, characterized in that the output frequency of theconverter (2) is at least 380 Hz.
 19. Method for operating at least oneelectrical load (7) in a subsea application using an electrical powersystem according to one of the preceding claims for power transmissionto a subsea power distribution system (5).
 20. Subsea remotely operatedvehicle with an electrical power system according to one of claims 1 to14 with at least one electrical load (7) being a propulsion system forthe subsea remotely operated vehicle, said propulsion system receivingpower from the subsea power distribution system (5).